The registry "IRW" is an abbreviation for "Imperial Romulan Warbird". It was used explicitly in reference to the Khazara in TNG: "Face of the Enemy" and is assumed to apply to other Warbird-like Romulan ships not otherwise identified. Casualty reports later appearing in Star Trek: Deep Space Nine began using the registry of "PWB", which might mean "Praetorian Warbird", possibly in reference to a subsection of the Romulan military.